When is a tempo effect a tempo distortion?

John Bongaarts, Griffith Feeney

Abstract

Our aim is to clarify the fundamental conceptual difference between a tempo effect and a tempo distortion and to answer the question of which uses of quantum and tempo measures lead to distortions. We conclude that tempo effects are widespread in period measures, but that the need for adjustment depends on the purpose of the indicator. When the objective is to measure current conditions, as is usually the case, two approaches exist. In the classical approach, current age specific event rates are assumed to reflect fully current conditions. In contrast, we argue that current rates and the tempo effects in measures derived from them do not in general reflect current conditions. When changes in the timing of demographic events shift rates schedules to higher or lower ages, constant conditions may imply changing rates. In that case, tempo effects are distortions that need to be corrected in order to get an accurate measurement of current conditions.

John BONGAARTS, Vice-President and Distinguished Scholar, Population Council, One Dag Hammarskjold Plaza, New York NY 10017, U.S.A. E-mail: jbongaarts@popcouncil.org.